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are you sure you are dropping the right tablespace?
It looks to me tablespace is alive and still has content in it.
Please do this...
select count(*) from dba_segments where tablespace_name = 'USERS';
What it shows?

Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.

OK then
select owner, constraint_name,table_name,index_owner,index_name
from dba_constraints
where (index_owner,index_name) in (select owner,index_name from dba_indexes where tablespace_name='YOUR_TABLESPACE IN UPPER CASE');

Then disable all these constraints and drop the tablespace. Later recreate the tablespace and reenable the constraints